Mendelian Genetics

Gregor Mendel born in 1822 Lived in what is now the Czech Republic Tended the garden of a monastery while living there as a monk

Mendel’s Plants Mendel worked with pea plants in his garden Mendel cultivated several stocks of pure-breeding pea plants These plants had a number of traits that occurred in one of two forms:

Green or yellow seed color

Round or wrinkled seeds

Tall or short plant height

And a variety of others...

Mendel’s Crosses After establishing his purebred peas, Mendel started his experiments Important vocabulary for understanding Mendel: P = parental generation F1 = first generation of offspring F2 = second generation of offspring

Mendel’s Crosses When he crossed tall plants with other tall plants, what type of offspring do you think he got? You guessed it… tall offspring!!! Tall + tall (P) = tall (F1)

Mendel’s Crosses When Mendel crossed short plants with other short plants, what do you think he got? Short offspring Short + short (P) = short (F1)

Mendel’s Crosses When he crossed tall plants with short plants, what do you think he got? Tall + short (P) = ?? He got ALL TALL plants in the F1 generation!!!

This was quite a surprise to him.

Why was Mendel surprised? Mendel was surprised because he had expected the traits to blend: Tall + short = medium Instead, the tallness of one parent had somehow masked the shortness of another parent in the offspring Tall + short = Tall????

Mendel’s 3 Conclusions 1) Mendel realized that biological inheritance is controlled by factors (genes) from each parent

Mendel’s 3 Conclusions 2) These genes came in two different forms. These forms are called alleles For example: the gene that controls the height of a plant has a tall form and a short form. The gene for flower color has a purple form and a white form.

Mendel’s 3 Conclusions 3) Some alleles are dominant while others are recessive When both types of alleles are present, dominant alleles mask (cover up) recessive alleles. Dominant allele Recessive allele

Mendel’s 3 Conclusions So each plant must have two alleles for each trait Short plants must have two alleles for short plant height (tt) Tall plants may have either two alleles for tall plant height (TT) or one allele for tall and one allele for short (Tt)

Tall/Tall (TT) or Tall/Short (Tt) Short/Short (tt)

P Generation F1 Generation The plants are homozygous--they have two copies of the same allele for plant height: TALL/TALL or SHORT/SHORT These plants are heterozygous--they have two different alleles for plant height: TALL/SHORT But they are all tall…why?

Genotype vs. Phenotype An organism’s genetic make- up = genotype An organism’s physical appearance = phenotype Possible Genotypes = TT, Tt, tt Possible Phenotypes = Tall, short

Genotype vs. Phenotype Do you know the genotype of a short plant? Genotype = tt What about the genotype of a tall plant? Genotype = TT or Tt (you can’t tell by its appearance)

Mendel’s Crosses What did Mendel do next? He crossed the tall F1 plants from the previous cross with each other to see what would happen… When he did this, he got more interesting results--what do you think happened??

Mendel’s Crosses The results of the F1 cross were: 75% tall plants 25% short plants Again Mendel was surprised because the short, recessive allele had returned--why did it reappear in the F2 when it wasn’t observed in the F1?

Mendel’s Conclusions Mendel knew that for there to be short plants in the F2 generation, those plants must have received two short alleles (tt) from their parents

Mendel’s Conclusions He figured out that each parent contributes one allele (for a certain trait) to the offspring Why does one parent only contribute one allele to its offspring--instead of two???

Mendel’s Conclusions Two Scenarios: 1) If this plant is homozygous tall (TT) it can only pass on a tall allele (T) to its offspring 2) If this plant is heterozygous tall (Tt) it can pass on either a tall allele (T) or a short allele (t) to its offspring

Mendel’s Conclusions Because this plant is short (tt) it can only pass on a short allele (t) to its offspring!

Parent Possible Gametes T Tall Plant TT T Tall Plant Tt T t

Parent Possible Gametes Short Plant tt t t

Mendel’s Conclusions Mendel called this Segregation: The two alleles from each parent are segregated from each other so that each gamete only carries a single copy of each gene

We can use Mendel’s Law of Segregation to figure out the genotypes and phenotypes that result from crosses between pea plants or any other organism…using a tool called Punnett Squares!

Punnett Square Male Gametes Female Gametes